Conventional noise reduction systems are disclosed, for example, in U.S. Pat. No. 4,064,530 entitled "NOISE REDUCTION SYSTEM FOR COLOR TELEVISION" issued to A. Kaiser et al. and in articles, "A Digital Noise Reducer for Encoded NTSC Signals" by R. H. McMann et al. and "Digital Techniques for Reducing Television Noise" by J. P. Rossi, published in the Journal of SMPTE, Vol. 87, March 1978 pages 129-133 and 134-140, respectively. In these disclosed systems, one to several frame portions of a video signal are recursively added to the present frame as long as there is a high inter-correlation, as for example in the case of a still object being shown. Such recursive addition serves to improve the S/N ratio. When the inter-frame correlation is relatively low, as in the case of a fast moving object being shown, either no addition is performed or fewer frame portions of the video signal are recursively added to the present frame. This is done to prevent the formation of an afterimage or lag that unavoidably accompanies recursive frame-to-frame addition for noise reduction.
Conventional noise reduction systems comprise a one-frame memory, two variable attenuators for attenuating the amplitudes of present and stored video signals, an adder for adding the stored video signal to the present video signal, and means for detecting motion between the present and stored frames to produce a frame-to-frame difference signal. The variable attenuators are ganged and introduce transmission factors (1-k) and k to the present and stored video signals, respectively, with the factor k being a function of the frame-to-frame difference signal. In practice, the variable attenuators must include plural sets of factors (1-k) and k. This necessity results in complicated and expensive variable attenuators.
It is, therefore, an object of this invention to provide a simple and inexpensive noise reduction system having a simplified attenuator.